1. Field of the Invention
This invention relates to an evaporative system in which evaporated fuel (hereinafter referred to as "evaporative gas"), produced in a fuel tank of an internal combustion engine, is temporarily adsorbed in a canister, and the evaporative gas thus adsorbed is discharged to an intake system, and more particularly to an evaporative system enabling a precise detection of a leakage in the evaporative system, and the invention also relates to a method of diagnosing the evaporative system.
2. Description of the Related Art
A so-called evaporative system is provided in order to prevent evaporative gas, produced in a fuel tank, from being discharged to the atmosphere. In this system, the evaporative gas is temporarily adsorbed by an adsorbent in a canister, and the thus adsorbed evaporative gas, together with fresh air drawn from an atmosphere port (drain) in the canister in accordance with an operating condition of an engine, is discharged or purged into an intake tube of the engine, and is burned.
However, the above evaporative system, though rarely, fails during the operation. For example, it is possible that a hole or a crack is formed in the fuel tank or an evaporative gas line extending between the fuel tank and the canister, and that a pipe of the gas line is dislodged out of place. In such a case, there is a possibility that the evaporative gas is not adsorbed by the adsorbent in the canister, but is discharged to the atmosphere. Among diagnosis items, the most important is a leakage diagnosis of the evaporative system, in which the leakage of the evaporative gas is detected during the operation, and a warning (or alarm) is given to the operator in order to prevent air pollution resulting from the failure of the evaporative system.
A method of diagnosing a leakage in an evaporative system is disclosed, for example, in Japanese Patent Unexamined Publication No. 6-10779. In this method, a shut-off valve, leading to a drain, is closed, and a purge control valve is opened, so that the pressure within the evaporative system is once made negative, and in this condition a purge valve is opened, and a leakage is detected from a pressure change in the evaporative system.
Japanese Patent Unexamined Publication No. 3-249366 discloses a method of diagnosing an evaporative system from a change in the air-fuel ratio when a purge control valve is opened and closed. In this method, a purge valve is opened and closed under a high load, and when a change in the air-fuel ratio is detected, the purge valve is again opened and closed under a low load, and the evaporative system is diagnosed from a change of the air-fuel ratio obtained at this time.
Japanese Patent Unexamined Publication No. 6-249095 (U.S. Pat. No. 5,353,771) discloses a method of diagnosing an evaporative system by controlling a purge valve at a duty corresponding to the amount of fuel remaining in a fuel tank.
In the above evaporative system leakage methods, whether the pressure within the sealed system is reduced (to a negative pressure) or increased, the diagnosis is made from a pressure change obtained when a leakage due to the pressure difference from the atmospheric pressure occurs. Therefore, if a pressure variation due to some factor develops inside or outside the evaporative system, the leakage can not be accurately diagnosed.
For example, when evaporative gas is being produced in the fuel tank, and particularly when the amount of production of the evaporative gas is large, the pressure within the system increases. Even during the diagnosis operation, the evaporation of the fuel continues, and therefore it is difficult to distinguish this pressure change from a pressure change due to the leakage, and this invites a gross error in the diagnosis result. Particularly in an environment in which the evaporation of the fuel is promoted (for example, when the amount of the fuel remaining in the fuel tank is small, or after the engine is operated for a long period of time, or when the engine is left for a long period of time in a hot climate), the temperature of the fuel itself is high, and therefore the pressure increase due to the production of the evaporative gas is large, and it is difficult to make a precise diagnosis. In the case of fuels different in volatility from each other, the rate of production of evaporative gas is different even if the remaining fuel amount is the same, so that the rate of rise of the temperature in the evaporative system is different, and this also is the cause of an erroneous diagnosis.
On the other hand, a change in the atmospheric pressure, which is an external environment of the evaporative system, is also a serious problem. With the same diameter of a leak, there is the difference in pressure change between a flatland and a highland at a height of above 2,000 m, and this is also the cause of an erroneous diagnosis. Thus, the diagnosis methods, utilizing a pressure change in the evaporative system, have suffered from problems that an error can be made in the diagnosis of the evaporative system by other pressure variation factors than a leakage, and that it is often difficult to effect the diagnosis itself.